Mammalian Target of Rapamycin (mTOR) is serine-threonine kinase and the catalytic subunit of mTOR Complex 1 (mTORC1). mTORC1 is a master regulator of cellular growth and viability, promoting protein synthesis, ribosome biogenesis, and mitochondrial biogenesis while inhibiting the catabolic process of autophagy. Not surprisingly, this pathway is dysregulated in cancer, and a number of well characterized tumor suppressors are negative regulators of the mTORC1 pathway. Regulation of such broad anabolic and catabolic pathways requires that mTORC1 acts as a monitor of the health status of the cell. Its activity is tightly regulated in cells by the levels of amino acids, growth factors, nd energy levels, as well as DNA damage and other cellular stresses. Of all of these inputs, amino acids are unique; they utilize an independent mechanism to activate the kinase and are absolutely required for activity, even in the presence of other activating stimuli. Despite growing knowledge of the upstream pathways that control mTORC1 activity, surprisingly little is known about how amino acids are sensed or how their levels signal to mTORC1. Elucidation of this crucial aspect of the mTORC1 pathway will be instrumental in providing new targets for pharmacological inhibition of mTORC1 while potentially minimizing toxic side effects. The goal of this project is to identify and characterize novel regulators of mTORC1 activity in response to amino acid levels. We have used a proteomic approach to identify proteins that interact with the machinery that activates mTORC1 in the presence of amino acids. In this proposal, we focus on the characterization of one particular interacting protein in the mTORC1 pathway. Additionally, this protein has been shown to be a cellular target of a Hepatitis B virus oncogene, possibly implicating a role for mTORC1 in both viral infection and Hepatitis B virus-associated hepatocellular carcinoma. To characterize this new protein, we propose to following aims: 1. Define the role of this new pathway member in the activation of mTORC1 by amino acids through its control of mTOR trafficking to the lysosome, where the kinase is activated, using knockdown experiments. 2. Identify proteins that interact with the new pathway member using immunoprecipitation followed by mass spectrometry in order to fully elucidate its function within the amino-acid sensing pathway. 3. Interrogate the role of the amino acid-sensitive mTORC1 pathway in Hepatitis B virus infection.